Glass compositions and method of making same



United States Patent GLASS COMPOSITIONS AND METHOD OF MAKING SAME Alexis G. Pincus, Worcester, Mass, assignor to American Optical Company, Southbridge, Mass, a voluntary association of Massachusetts N0 Drawing. Original application July 27, 1948, Serial No. 40,963, now Patent No. 2,716,059, dated August 23, 1955. Divided and this application May 18, 1955, Serial No. 509,356

3 Claims. (Cl. 106-47) This invention relates to glass compositions and has particular reference to glasses which have a low index of refraction and a low or controllable optical dispersion and process of making the same.

This application is a division of applicants co-pending application, Serial Number 40,963, which was filed July 27, 1948, and which issued August 23, 1955, as Patent Number 2,716,069. Other divisional applications are Serial Number 509,355 and Serial Number 509,357, both filed May 18, 1955.

One of the principal objects of the invention in its broad aspect is to provide glasses whose index of refraction is considerably below known commercial glasses, whose index of refraction and reciprocal relative dispersion properties may be varied with respect to each other over a broader range than has heretofore been available, whose chemical durability will be adequate to meet the requirements of particular uses, and which are readily workable and afford ease of fabrication and refabrication,

Another object is to provide glasses of the above nature which may be melted and worked at very low temperatures lying in the region between known commercial glasses and. organic plastics and process of making the same.

Another object is to provide glasses of the above nature whose indices of refraction may be more positively controlled and over a wider rangethan known commercial glasses.

Another object is to provide glasses of the above nature whose dispersive properties may be more positively controlled and over a wider range than known commercial glasses.

Another object is to provide glasses of the above nature which include as essential ingredients fluorine and oxygen in varying ratios as anions combined with selected positive elements as cations in controlled related proportions depending upon the resultant characteristics desired.

More specifically, the object of the invention is to provide glasses of the above nature which include, as an essential ingredient thereof, beryllium fluoride.

Another more specific object is to provide glasses of the above nature which are formed from blends of metaphosphates with beryllium fluoride.

Another object is to provide glasses of the above character which utilize as fully as possible available and relatively economical raw materials.

Another object is to provide glasses of the above nature possessing characteristics which make possible new fabricating techniques and new applications in coatings, impregnants, cements, enamels, glazes, etc.

Another object is to provide novel processes of obtaining all of the above objects and advantages.

Other objects and advantages of the present invention will become apparent from the following description and it will be apparent that many changes may be made in the compositions and processes set forth herein without departing from the spirit of the invention as expressed 'in the accompanying claims. I, therefore, do not wish 2,991,363 Patented Aug. 25, 1959 ICC to be limited to the exact compositions and processes disclosed as the preferred forms only have been given by way of illustration.

Some glasses possessing the characteristics set forth herein are known in the art but such known prior art glasses did not possess sufficient resistance to weathering and ease of fabrication and refabrication to render them commercially practical. Some suggestions were made of the possibilities in beryllium fluoride glasses from theoretical reasoning by V. M. Goldschmidt, Skrifter Norske VidenskapsAkad., Oslo (1926), and very limited research was conducted in the field of beryllium fluoride glasses by Dr. George Heyne, Angewandte Chemie, vol. 46, Germany (1933), but neither in any way achieved or taught the broad scope or findings and results obtained by the present invention. Heynes contribution to the art merely included glasses which he stated were not stable for practical applications, especially in optics The beryllium fluoride glasses must, where remaining clear for a long time is required, be protected from moist air or embedded in suitable material. Glasses of the teachings of the present invention do not require this protection and also, in certain compositions taught, are more durable than many glasses which are commonly used in optical applications.

Such known prior art beryllium fluoride glasses embodied compositions utilizing a high proportion of relatively expensive ingredients and failed, in general, to provide the art with any definite knowledge as to control of refractive indices and dispersive properties and at most embodied a very narrow range of compositions as compared with the present invention and offered little, if anything, of commercial interest.

Attempts have also been made to produce lowered refractive index and dispersion by introducing fluorine into an alkali borosilicate base, free from bivalent oxides, resulting in the optical fluor-crowns containing up to about 7% fluorine by weight. These glasses did not provide a much from V=67. Compared to the glasses of the present invention they are difiicult to melt, give poor quality yields, and are of inferior chemical durability.

Glasses of the present invention open an entirely new field of research as to optical systems and lens design in general and lend themselves to several different applications. For examples of particular uses: such glasses may replace crystals in highly corrected lens systems; may be used as an intermediate medium for supporting the elements of different lens systems in more positive relation with each other and at the same time replace the conventional air spaces of the elements of such systems with a less abrupt interfacial transition in refractive indices; and will aflord other design possibilities which can take advantage of their unique optical characteristics.

The invention is directed particularly to the combining of BeF (beryllium fluoride) or equivalent substituents, (PO (the metaphosphate radical) or equivalent substituents, and R+ (an ingredient or ingredients of the metallic cations group) in such a manner as to obtain glasses having low or variable and controllable indices of refraction, low or variable and controllable optical dispersions, controlled as to stability and resistance to chemical attack and weathering, controllable characteristics as to melting and softening properties, introducing ease of fabrication and possessing practical working and remelting characteristics.

Another method of describing the possible constituents and their relative proportions is by the generalized formula:

caesium 05+ cadmium (cd ,*1ead (Pb thallium ('Il and CH) strontium (Sr or-the like;

B represents the network-forming cations of the -type beryllium (Be++-), aluminum (Al silicon (Si z-irconium (Zr phosphorous (P boron (B s ulphur (S nitrogen (N carbon (C titanium (Ti or the like. It is understood that certa n-Cations, such as aluminum (Al are able to function as both hole-fillers and network-formers depending onthe chemical balance among the remaining ions; '0 represents the,

oxygen anion, F represents the fluorine anion, F.

In this generalized formula the sum of the anions prescut is always taken as unity and the relative proportions of cations present refer to the unit anion. Thus, if fluorine is the only anion it will be given the subscript unity. Where both fluorine and oxygen are present, the fluorine will differ from unity by the proportion x of oxygen present and the fluorine will have the subscription l'-x resulting from subtracting the number represented by x from unity. If other anions such as chlorine, bromine or sulphur are introduced, they will be included in the total within the parenthesis, but the sum of the anions will still be reduced to unity.

The subscripts m and n, respectively, indicate the relative numbers of ions of type A and type B per unit anion.

Examples I. Berryllium fluoride (BeF glass would be expressed,

as Be F.

II. A useful glass composition has been found'tobe made from the batch:

Percent byweight Beryllium fluoride 40 Cryolite 40 Sodium metaphosphate it being understood that in this formula or wherever, 116-.

ferred to herein, the metaphosphate of sodium can be The preceding values may be derived as follows: Since the chemical formula for cryolite is Na AlF the formula for sodium metaphosphate is NaPO and the formula for beryllium fluoride is BeF and the atomic weights for the elements contained therein Na=22.99, Al 26.97, F=l9.00, 0:16.00, P= 30.98 and Be;9.02, it can readily be ascertained that the molecular weight ofcryolite is equal to 209.96, the molecular weight of sodium metaphosphate is equal to 101.98 and the molecular weight of beryllium fluoride is equal to 47.02.

It follows, therefore, that since cryolite constitutes 40% by weight of the above batch, the actual weight of the sodium contributed by the cryolite will be equal to 3 22.99 40/209.99, or 13.14%; the actual weight of, the aluminum in the batch willbe equal to 26; 97' 40/209.96, or 5.14%; and the actual weight of the fluorine contributed by the cryolite will be equal to 6 19 0 -40/209.96, or 21.72%. Therefore, the Na contributed by the cryolite-isequal to 1314/2299 or. 0.571 mole.

Since the sodium metaphosphate constitutes 20% by ht o t e ba t actua W t o he di m contributed thereby will be equal to 22,99 20/101.98 or 4.51%. metaphosphate will be equal to 4.5l/22.99 or 0.196 mole. The total Na in theba'tch due to both the cryolite Thus, the Na in the batch due to the sodium 4 and the sodium metaphosphate will be 17.65% and accordingly, will be equal to 0.57l+0.196 or 0.77 mole.

In like manner, values for the other elements in the batch may be calculated; The aluminum in the batch, as pointed out above, has an actual weight of 5.14% and when this figure is divided by 26.97, it will give 0.19 mole.

The actual. weight of fluorine in the cryolite equals 21.72%, as given above, and the actual weight of the fluorine in the beryllium fluoride equals 2x 19 40/ 57.02, or 32.33%. The total fluorine in thebatch due to the. cryolite and the beryllium fluoride is,--therefore, 54.05% and when divided by the atomic weight of fluorine gives 2.84 moles.

The actual weight of the beryllium in the batch is equal to 9.02/47.02x40, or 7.67% and when divided by 9.02 gives 0.85 mole.

Thephosphorusin; the batch is equal to or 6.08%; and when this fi'gure is divided by 30.98,;it

gives 0.20 mole. Likewise, the oxygen is equal to 3 16.00 20/101.98, or-9.41%-; and when divided by 16.00 gives.0.-59 mole.

Applying this information to the general formula A B,,(O,',F -it-will appear a-s follows-z with thelast two valuesadding up to 3.43. Considering the O+F sum asunity, the other values of the formula may be related to unity by-dividing each by 3.43. This will give The phosphorus cation P has been found to be a particularly useful member; of the B group because it bringsabout a compatibility between oxygen and fluorine anions which greatly promotes glass-forming tendencies and inhibits crystallization or devitrification.

Also within the B group it has been found possible to replace some of the phosphorus cations (P with nitro-- gen (N and carbon (0 with marked improvement in chemical durability of the glasses and unusual optical properties resulting.

By this form of representation the invention is directed particularly to the combining of B network-forming cations with suflicient A non-network forming cations to assure homogeneous vitreous characteristics and with balanced proportions of fluorine and oxygen (and other anions such as mentioned above) in such a manner asto obtain true homogeneous glasses having low or variable. and controllable, indicesof refraction, low or variable and controllable optical dispersions, controlled as to stability and resistance to chemical attack and weathering, con-v trollable characteristics as to melting and softening prop-- erties, introducing easeof fabrication and possessing practical working and remelting characteristics.

Pure beryllium fluoride (BeF glass has been found to possess extraordinarily low refractive index and optical dispersion. These-properties have been found to be as follows:

ND=1.27475 N =1.27 649. N =1.27392 i J r Unfortunately for its usefulness in optical systems this glass is extremely hygroscopic and it is the essence of this invention to teach the obtaining of glasses of the above nature, namely glasses having low refractive index and optical dispersion, with improved chemical durability.

As discussed more completely in the parent application other compounds have been combined with beryllium fluoride in controlled proportions whereby a homogeneous melt may be obtained and which can be cooled to room temperature and reworked without loss of homogeneity.

Although, as shown in the analysis contained in the parent application, the index of refraction increases with theaddition of sodium fluoride and sodium metaphosphate to the beryllium fluoride, several different indices of refraction and Nu values may be obtained by shifting the related proportions of the ingredients while increasing the durability of the glass.

There is no upper limit to the sodium metaphosphate content for glass-forming characteristics but when the sodium metaphosphate content is above 80% the resultant glasses tend to be hygroscopic.

They, however, may meet the requirements of particular uses, such as for optical elements which may be protected by imbedding them in a resin or by sealing them within a lens system, for use as cements, or other uses. Several interesting findings were as follows:

" a. Chemical durability improves as NaF replaces BeF and even more as NaPO increases. There is an optimum in durability between 40 and 80% of NaPO b. Refractive index increases slowly as NaF replaces BeF though at a progressively slower rate as NaPO increases to about 70%. The most important factor in establishing refractive index level is NaPO content. This can be shown to be a function of the ratio of fluorine to oxygen in the atomic formula method of expressing composition.

a. Nu values in general are higher as the Belcontent is increased and the NaPO content is decreased and vice versa, but there are exceptions. A maximum dispersion seems to exist around 30% NaPO and a minimum around 80to90%NaPO d. Meltability is controlled by the proper balance of beryllium fluoride for each sodium metaphosphate level.

Species of glass according to the present invention The observations teach as the widest possible range the following:

Range of percentages by weight Beryllium fluoride (BeF c Approximately to 50 Sodium metaphosphate (NaPO Approximately 50 to 90 The sodium metaphosphate may be replaced in whole or in part by aluminum metaphosphate.

A particularly interesting type of substitution is that in which the sodium metaphosphate is replaced not only by other metaphosphates but by salts supplying other radicals in place of the metaphosphate radical, for ere ample, carbonate (C09 nitrate (NO sulphate (804)?" and the like. In certain cases, it has even been found possible to introduce the hydroxyl anion (OH). In all of these cases, sodium has been used as the cation constituents of the salt but has been found to be replaceable by calcium, potassium, zinc and the other A group elements.

Glasses of this unusual type require theme of some relatively novel raw materials for glass making. Beryllium fluoride is available in the form of lumps of high purity beryllium fluoride glass from manufacturers of beryllium chemicals. It has also been obtained in a less pure powdered form.

V The choice of a refractory to hold this type of melt depends on how high the fluorine content is compared to the phosphate. At the highest fluorine contents platinum seemed most desirable and was not attacked directly by the melts, but it was. found that care had to be taken not to let metallic iron or other metals from alloys come in contact with platinum and the melt as the metal was fluxed by the fluoride-phosphate and in turn attacked the platinum. Fused silica crucibles, graphite and carbon are also practical.

In the high phosphate range ordinary ceramic crucibles (aluminum silicates) have been found to be satisfactory, and in some cases even preferable to platinum.

The choice of melting cycle is also determined by the relative amounts of fluorine and oxygen, that is of fluorides and metaphosphates. Most of these compositions, however, could be melted between 1600 and 1900" F. The higher fluorine glasses fumed considerably during this melting stage particularly if the raw materials are damp or contain water of constitution; therefore, the time at this upper melting temperature should be kept as short as possible and for experimental melts of 50 to 250 grams about 5 minutes was ordinarily adequate for complete melting. The lower fluorine glasses are less critical and may be held for longer periods in the melting range. The furnace temperature should then be lowered as rapidly as possible to about 1200 F., depending upon the viscosity and the devitrification tendency of the particular formula, and the melt held at that temperature for about a half an hour or long enough to permit it to calm down and get rid of its fuming and bubbles. Certain of the compositions could be held at temperatures as low as 850 F. without devitrifying and still poured fluidly. Because of the fuming and the extremely fluid nature of these glasses they tend to be quite striated and it is necessary to stir them at the lower temperature. The melts can be poured into graphite or graphitized iron moulds and then annealed. Because of the wide range of compositions taught no specific annealing temperature can be mentioned although 450 F. has been found satisfactory for most of the more desirable formulas. After annealing to release strain and cooling slowly at room tem perature, glasses are obtained which can be ground and polished and otherwise handled like conventional glasses. They possess the extraordinary property of being workable at extremely low temperatures, intermediate between the working temperature of normal glasses and organic plastics. The glasses with higher fluorine ratios (fluorine content of 0.50 or higher) have been worked by pressing at temperatures as low as 600 F. Glasses with a durability rating of 3 or 4 can be polished by conventional means but it has been found helpful to use a solution of ethylene glycol and water as the liquid suspending the rouge.

These glasses exhibit other unusual properties such as extremely high thermal expansion, low softening temperatures and unusual transmissions in the ultra-violet and infra-red regions.

Therefore, from the above teachings, various types of homogeneous structures and specific glass-forming compositions can be produced which permit control of the controlled amounts of fluo'rin and' ojqjlgeii in ya ryingfatios as anions m'aseiectea"pbtitiveelemettts asfcations in oontnolled related proportions depending upon 'the'resultantcharacteristicsHesiredf formulas; "final analysis, percentages etc. given above are bywa of illustration only and should notrbelimitiye of the inventionlexcept in so far as they are specifically recited in" the appending claims From the foregoing, it will be seen" tliatl have produced ri e ans ethod of alsimple and'efiicient nature that will produce allof theobjects" and advantages of the p i r g a a H a Ewing-described my inyention, I claim: 7

method 'of' niaiiufacture comprising the steps of preparing a glass batchconsi'sting essentially of approximately to 50 by weight of beryllium fluoride (Belf l; approximately 20' tof40%' by weightfof sodium metaphosphate (N a oa), and the balance approximately 30. to 50% by weight of aluminum metapho'sphate (Al (P051 heatingslaid' batch at a temperature and for a period of time sufficientto inelt the' ingredients thereof and 'prodniiea homogeneous glassy structure; annealing said structure at a lowei'ternperature to relieve strain therein, and slowly cooling said structure to room ternpera l 'i, to thereby produce a clear durable transparent o c ass- I Th? 19 9 m f fi l comprising h Steps 8. of preparing a glass batch consisting essentially of approximately 50% by weig'hzt erserynimn fluoride (BeF approximately 20% by weight of sodium metaphosphate (NaPO and approximately by weight of aluminum metaphosphate A1(PO ),heating said batch at a'ternp'eratureand fora period of timesufiicient to melt the ingredients thereof and produce "a' homogeneous glassy s 'tructuriej aiinealingfsaid? structure at a lower tempera-' ture torelieve'strain' therein, and slowlycooling said:

structure to room temperature; to thereby produce a clear durable transparentfoptical glass.

3. I'he' m'ethod of manufacture'comprising the steps of preparing'aglass batch consisting essentially of approxim-ately' 10% by weight of beryllium fluoride (BeF approximately 40% by weight of sodium metaphosphate (NaPO "and approximately by weight'of aluminum metaphosphate (Al(PO heating said batch at aternperature and 'for a period of timesufficient' to melt the ingredients thereof and produce a homogeneous glass structure; annealing said structure at a lower tempera ture to relieve "strain therein,' and" slowly cooling said structure to room temperature, to thereby produce a clear durable transparent optical glass.

Refe ensesfiitedi the fi es)? l3i$P91Bl S 'IA-TES PATENTS 2,481,700 Sun et a1 ;'se t. 13, 1949 2 716,069 J' Fi APg- 5 ER RE E EN Blau et all: The Glass Industry, February 1935, page51. 

1. THE METHOD OF MANUFACTURE COMPRISING THE STEPS OF PREPARING A GLASS BATCH CONSISTING ESSENTIALLY OF APPROXIMATELY 10 TO 50% BY WEIGHT OD BERYLLIUM FLUORIDE (BEF2), APPROXIMATELY 20 TO 40% BY WEIGHT OF SODIUM METAPHOSPHATE (NAPO3), AND THE BALANCE APPROXIMATELY 30 TO 50% BY WEIGHT OF ALUMINUM METAPHOSPHATE (AL(PO3)3), HEATING SAID BATCH AT A TEMERATURE AND FOR A PERIOD OF TIME SUFFICIENT TO MELT THE INGREDIENTS THEREOF THE PRODUCE A HOMOGENEOUS GLASSY STRUCTURE, ANNEALING SAID STRUCTURE AT A LOWER TEMEPRATURE TO RELIEVE STRAIN THEREIN, AND SLOWLY COOLING SAID STRUCTURE TO ROOM TEMPERATURE, TO THEREBY PRODUCE A CLEAR DURABLE TRANSPARENT OPTICAL GLASS. 